1. Field of the Invention
The present invention relates to an optical head used in optical information processing, optical communication or the like and an optical recording and reproducing apparatus using the optical head.
2. Description of the Related Art
Recently, a digital versatile disc (DVD) has attracted attention as a high-capacity optical recording medium because it can record digital information in a recording density which is about 6 times as high as a compact disc (CD). However, a further high-density optical recording medium is demanded as capacity of information becomes large. Here, in order to realize a density higher than the DVD (wavelength is 660 nm and numerical aperture (NA) is 0.6), it is necessary to use a light source emitting a light having shorter wavelength and to further increase the NA of the objective lens. For example, when blue laser having a wavelength of 405 nm and an objective lens having NA of 0.85 are used, a recording density which is 5 times as high as the DVD can be attained.
However, since the high-density optical recording medium apparatus using the blue laser has very strict reproducing and/or recording margin, in other words, a permissible level for a fluctuation of characteristic in reproducing or recording is limited very strictly, aberration generated by a fluctuation in the base-substrate thickness of an optical recording medium becomes a problem. It is to be noted that the wording “reproducing and/or recording” means “at least one of reproducing and recording”, in the specification, to simplify the description.
In relating to this problem, Japanese Patent Laid-open Publication No. 2000-131603 discloses an optical head which aims to carry out reproducing and recording operations while correcting aberration due to a fluctuation in the base-substrate thickness of an optical recording medium.
One example of the above conventional optical head is described with reference to the drawing.
FIG. 9 is a schematic view showing a constitution of the conventional optical head. In FIG. 9, reference numeral 91 designates a light source, reference numeral 92 designates a diffraction grating, reference numeral 93 designates is a collimator lens, reference numeral 94 designates a polarized beam splitter, reference numeral 95 designates a ¼ wavelength plate, reference numeral 96 designates a group of aberration correcting lenses, reference numeral 97 designates an objective lens, reference numeral 98 designates an optical recording medium, reference numeral 99 designates a focusing lens, reference numeral 100 designates a multi-lens and reference numeral 101 designates a light detector.
The light source 91, which is a semiconductor laser, serves as a light source that outputs coherent light for use in recording and reproducing to a recording layer of the optical recording medium 98. The diffraction grating 92 has a structure in which a concave/convex pattern is formed on a surface of a glass substrate, and serves an optical element which divides an incident beam into three beams so as to allow detection of a tracking error signal through a so-called three beam method. The collimator lens 93 is a lens which converts diverged light emitted from the light source 91 to parallel light rays. The polarized beam splitter 94 is an optical element which has different transmittance and reflection factor depending on incident polarized light, and is used for separating light. The ¼ wavelength plate 95 is made from a birefringence material, and serves as an optical element that converts linearly polarized light to circularly polarized light.
The group of aberration correcting lenses 96, which is used for correcting spherical aberration that occurs when the base-substrate thickness of the optical recording medium 98 is different from a predetermined standard value, is constituted by a group of concave lenses 96a and a group of convex lenses 96b as well as a uniaxial actuator, not shown. And, by changing the distance between the group of concave lenses 96a and the group of convex lenses 96b, it becomes possible to correct the spherical aberration. The above-mentioned standard value is, more preferably, determined based on an optimum design base-substrate thickness as a thickness of the base-substrate of the optical recording medium 98. The group of aberration correcting lenses 96 will be described later in detail
The objective lens 97 is a lens for converging light on a recording layer of the optical recording medium 98. The focusing lens 99 is a lens used for converging light reflected from the recording layer of the optical medium 98 onto the light detector 101. The multi-lens 100 has a cylindrical surface as its light incident face, and its light-releasing face forms a rotation symmetrical face with respect to the lens light axis so that astigmatism, which allows the detection of a focus error signal with respect to incident light through a so-called astigmatism method, is given. The light detector 101 receives light reflected by the recording layer of the optical recording medium 98 to convert the light to an electric signal.
The following description will discuss operations of the optical head having the above-mentioned arrangement. Linearly polarized light, emitted from the light source 91, is divided into three beams by the diffraction grating 92, and the three divided light beams are converted to parallel light rays by the collimator lens 93. The resulting parallel light rays are allowed to pass through the polarized beam splitter 94, and made incident on the ¼ wavelength plate 95 so that the linearly polarized light is converted into circularly polarized light. The circularly polarized light that has passed through the ¼ wavelength plate 95 is made incident on the group of aberration correcting lenses 96. In this case, in order to correct spherical aberration that occurs when the base-substrate thickness of the optical recording medium 98 deviates from an standard thickness, the incident parallel light rays are converted to diverging light and converging light by changing the distance between the group of concave lenses 96a and the group of convex lenses 96b that constitute the group of aberration correcting lenses 96. Then, the converted light is made incident on the objective lens 97 so that spherical aberration is generated in proportion to a degree of divergence or a degree of convergence of the incident light, and is converged on the optical recording medium 98.
Here, since light having wave aberration capable of correcting the wave aberration occurring upon deviation in the base-substrate thickness of the optical recording medium 98 from the standard base-substrate thickness is converged thereon by the objective lens 97, a light spot that is free from aberration, that is, a light spot that is limited to the diffraction limit, is formed on the optical recording medium 98. Next, the circularly polarized light, reflected from the optical recording medium 98, is allowed to pass through the group of aberration correcting lenses 96, and is input to the ¼ wavelength plate 95, then is converted to linearly polarized light in a direction orthogonal to the linearly polarized light that has been emitted from the light source 91. The linearly polarized light, converted by the ¼ wavelength plate 95, is reflected by the polarized beam splitter 94, and converged by the focusing lens 99 without returning to the light source 91 so that astigmatism is given to the light made incident by the multi-lens 100 and the resulting light is converged on the light detector 101. The light detector 101 outputs a focus error signal that indicates a focused state of light on the optical recording medium 98, and also outputs a tracking error signal that indicates an irradiation position of light.
Here, the focus error signal and the tracking error signal are detected by known techniques such as an astigmatism method and a three beam method. Based upon the focus error signal, a focus control device, not shown, controls the position of the objective lens 97 in the light axis direction so that the light is always converged on the optical recording medium 98 in the focused state. Moreover, based upon the tracking error signal, a tracking control device, not shown, controls the position of the objective lens 97 so that light is converged on a desired track on the optical recording medium 98. Furthermore, information recorded on the optical recording medium 98 is also obtained by the light detector 101.
Here, the following description will discuss the spherical aberration correcting operation that is available by the use of the group of aberration correcting lenses 96, in detail. When the distance between the group of concave lenses 96a and the group of convex lenses 96b constituting the group of aberration correcting lenses 96 is narrowed, the parallel light rays are converted to diverging light, and when the distance is widened, the parallel light rays are converted to converging light. In other words, by changing the distance between the group of concave lenses 96a and the group of convex lenses 96b, it is possible to generate light rays having power components with different codes. Here, in the case when light having a power component is made incident on the objective lens 97, spherical aberration occurs in the light converged by the objective lens 97, and since the code is dependent on the code of the incident power component, it is possible to correct the spherical aberration that occurs upon deviation of the base-substrate thickness of the optical recording medium 98 from a standard base-substrate thickness by using this spherical aberration.
With this arrangement, since the spherical aberration caused by the deviation in the base-substrate thickness of the optical recording medium 98 can be corrected by using the group of aberration correcting lenses 96, it is possible to carry out stable reproducing and recording operations.
In the optical head having the above-mentioned conventional arrangement, however, no description has been given to a light-quantity detection device that is required to control the quantity of light released from the light source 91, with the result that a problem arises due to the position of this light-quantity detection device. Referring to FIG. 10, the following description discusses this problem in detail. Here, only the points in which an optical head shown in FIG. 10 is different from the optical head of FIG. 9 are that a mirror and a light-quantity detection device are further installed and that the ¼ wavelength plate is placed between the mirror and the objective lens; except for these points, it has the same arrangement as the optical head of FIG. 9. Therefore, in FIG. 10, the same parts as those of the optical head of FIG. 9 are used, unless otherwise indicated, and those components indicated by the same reference numerals have the same functions, unless otherwise indicated.
In FIG. 10, reference numeral 201 is a mirror, reference numeral 202 is a condenser lens and reference numeral 203 is a light-source light-quantity controlling light detector. Here, the light-quantity detection device is constituted by the condenser lens 202 and the light-quantity controlling light detector 203.
The mirror 201 is an optical element that reflects incident light to direct the resulting light to the optical recording medium 98, and with respect to certain linearly polarized light, transmits 5% thereof, while reflecting 95% thereof, and with respect to linearly polarized light orthogonal to the above-mentioned linearly polarized light, reflects 100% thereof.
The following description will discuss operations of the optical head having the above-mentioned arrangement. Linearly polarized light, released from the light source 91, is divided into three beams by the diffraction grating 92, and the three divided light beams are converted to parallel light rays by the collimator lens 93. The light, converted into the parallel light rays, are allowed to pass through the polarized beam splitter 94, and made incident on the group of aberration correcting lenses 96. In this case, in order to correct spherical aberration that occurs when the base-substrate thickness deviates from a standard value, the incident parallel light rays are converted to diverging light and converging light by changing the distance between the group of concave lenses 96a and the group of convex lenses 96b that constitute the group of aberration correcting lenses 96; thus, the converted light is made incident on the mirror 201 so that one portion (5%) thereof is allowed to transmit, while most (95%) of it is reflected, and changed in its advancing direction to the optical recording medium 98. This reflected light is made incident on the ¼ wavelength plate 95 to be converted from linearly polarized light to circularly polarized light; thus, this circularly polarized light is made incident on the objective lens 97 so that spherical aberration is generated in proportion to a degree of divergence or a degree of convergence of the incident light, and is further converged on the optical recording medium 98. Here, since light having wave aberration capable of correcting the wave aberration occurring upon deviation in the bas-substrate thickness of the optical recording medium 98 from the standard thickness is converged thereon by the objective lens 97, a light spot that is free from aberration, that is, a light spot that is limited to the diffraction limit, is formed on the optical recording medium 98.
Next, the circularly polarized light, reflected from the optical recording medium 98, is inputted to the ¼ wavelength plate 95, and converted to linearly polarized light in a direction orthogonal to the linearly polarized light released from the light source 91. The linearly polarized light converted by the ¼ wavelength plate 95 is all reflected by the mirror 201, allowed to pass through the group of aberration correcting lenses 96, and reflected by the polarized beam splitter 94 and further converged by the focusing lens 99 without returning to the light source 91 so that astigmatism is given to the light made incident by the multi-lens 100 and the resulting light is converged on the light detector 101.
The light detector 101 outputs a focus error signal that indicates a focused state of light on the optical recording medium 98, and also outputs a tracking error signal that indicates an irradiation position of light. Here, the focus error signal and the tracking error signal are detected by known techniques such as an astigmatism method and a three beam method. Based upon the focus error signal, a focus control device, not shown, controls the position of the objective lens 97 in the light axis direction so that the light is always converged on the optical recording medium 98 in the focused state. Moreover, based upon the tracking error signal, a tracking control device, not shown, controls the position of the objective lens 97 so that light is converged on a desired track on the optical recording medium 98.
Furthermore, information recorded on the optical recording medium 98 is also obtained by the light detector 101. Moreover, the light that has passed through the mirror 201 is converged on the light-source light-quantity controlling light detector 203 by the condenser lens 202, and the light-source light-quantity controlling light detector 203 outputs an electric signal corresponding to the quantity of light released from the light source 1.
The necessity of the above-mentioned light-quantity detection device is explained as follows: Since the light source 91 is formed by a semiconductor laser, the light source 91 has a temperature rise when it continues to output light, with the result that the quantity of light to be outputted from the light source 91 tends to vary even when the current used for controlling the light source 91 is constant. Therefore, by detecting one portion of the light released from the light source 91, it becomes possible to control the quantity of light released from the light source 91.
However, in the case when the signal detected by the light-quantity detection device is varied independent of the quantity of light from the light source 91, a serious problem is raised. For example, even in the case of constant quantity of light from the light source 91, when the signal outputted from the light quantity detection device becomes smaller, the light source 91 is controlled so as to release a greater quantity of light, with the result that a great quantity of light is released during a reproducing operation of the optical recording medium 98 to cause erroneous erasing of information recorded in the optical recording medium 98. In contrast, even in the case of constant quantity of light from the light source 91, when the signal outputted from the light quantity detection device becomes greater, the light source 91 is controlled so as to release a smaller quantity of light, with the result that the quantity of light fails to reach a sufficient quantity required for recording during a recording operation on the optical recording medium 98 to cause an insufficient recording process. In other words, a serious problem is raised unless the signal detected by the light-quantity detection device varies in response to the quantity of light released from the light source 91.
FIG. 11 schematically shows light to be made incident on the objective lens 97 when the group of aberration correcting lenses 96 is driven to correct spherical aberration. In FIG. 11, in the case when the base-substrate thickness of the optical recording medium 98 is thicker than a standard thickness, the distance between the group of concave lenses 96a and the group of convex lenses 96b becomes wider so that the light is made incident on the objective lens 97 as converged light. This state is indicated by a solid line. In the case when the base-substrate thickness of the optical recording medium 98 is thinner than the standard base-substrate thickness, the distance between the group of concave lenses 96a and the group of convex lenses 96b becomes smaller so that the light is made incident on the objective lens 97 as diverged light. This state is indicated by an imaginary line. Here, it is supposed that the light to be used in the light-quantity detection device is located at position A in FIG. 11.
In FIG. 10, an aperture (not shown), which is used for controlling the quantity of transmitted light, is formed between the mirror 201 and the condenser lens 202, and this is schematically indicated as an aperture 110H (opening) in FIG. 11. This aperture 110H is provided by forming a hole (opening) in a plate member 110. The member 110 having aperture 110H may be a hold member for holding the group of convex lenses 96b. 
As shown by FIG. 11, although the group of aberration correcting lenses is designed so as to make the quantity of incident light onto the objective lens 97 constant independent of the location of the group of concave lenses 96a while the group of concave lenses 96a is shifted to correct spherical aberration, the light to be made incident on the light-source light-quantity controlling light detector 203 is shield by the member 110 having the aperture 110H on the peripheral portion thereof depending on the position of the group of concave lenses 96a, with the result that the quantity of light to be detected by the light-source light-quantity controlling light detector 203 is varied.
In other words, although the quantity of light of the light source 91 is not changed, the quantity of light to be made incident on the light-source light-quantity controlling light detector 203 is varied, as described above, with the result that a signal is outputted as if it were derived from a change in the quantity of light in the light source; consequently, problems are raised in that recorded information is erased during reproducing, and in that insufficient recording is caused due to a failure in outputting a sufficient quantity of light upon recording.